Visit your dashboard, press on the “Upgrade to Premium” button and follow the wizard to complete the payment. The upgraded site will be moved to the Premium Sites section of your dashboard where you will see a link to download the premium version and a new premium API key.
You will not need to reconnect and authorize your Google Analytics property.
Install and activate the premium version and copy paste the premium API Key to the configuration tab of the plugin.
Please deactivate the free version to avoid any conflict.